Use of composites, such as carbon fiber reinforced plastics (CFRP), is becoming more common in aerospace applications for a many reasons including that composites allow designers to improve structural performance and reduce aircraft weight. Generally, composites include a reinforcing material embedded in a matrix of a polymeric composition. The reinforcing material may be oriented ply, woven fabric, particulates, or another form that provides suitable performance as a reinforcing material. The reinforcing material may be fiber glass, carbon fiber or other suitable reinforcement. The more typical polymer matrix material in aerospace applications is an epoxy resin but other polymers may also be suitable.
While composites have the advantage of low weight and high strength, the electrical properties of composites are very different from the metals that the composites often replace in aerospace applications, such as aluminum. Aerospace structural metals are generally highly conductive compared to composites. As a result, composites cannot distribute current and heat from a lightning strike (typical about 100,000 Amperes at 50,000 Volts) as quickly as metals.
Several techniques have been developed to provide lightning protection for composite structures. In general, these require the addition or incorporation of metallic conductors into composite exterior (or “near exterior”) surfaces of the aircraft (such as skin panels on the wings and fuselage) capable of distributing and diverting current away from flight critical areas and underlying aircraft components. The addition of metallic conductors has been approached in two ways: the use of metallic appliques applied in post-assembly operations and the integration of metal foil into the composite skin during composite lay-up. Applique-based systems have alternate layers (sheets) of dielectric and conductive materials applied over the composite structure surface and secured to the surface with an adhesive. This both insulates underlying aircraft components from a lightning strike and provides a conductive path for rapid distribution and dissipation of lightning current and heat. The conductive layer is typically aluminum or copper foil, either in solid or expanded (open mesh) form. The alternating dielectric layers can be polyimides, fluoropolymers or similar heat resistant, durable, high dielectric strength materials. The integrated foil systems may typically include copper or aluminum foil in either solid or expanded mesh form that is laminated into and co-cured with the composite material. This type of system provides a conductive path for diversion and distribution of lightning current which, in combination with special fasteners and other features, provides a degree of lightning protection for the composite structure.